Clever solutions to cutting energy bills aren’t always obvious. First of all you have to decide whether to replace expensive energy with cheap energy or to somehow figure out a way to use less energy in the first place.

It gets harder when the property owner isn’t keen on layering their sandstone heritage building with PV panels and there is little scope for weaning the elderly occupants off the comfortable climate produced by the energy-hungry air-conditioning system.

A lot of energy consultants would be left scratching their heads over that one, until they realised they were standing not far above an absolutely massive body of water that never deviates from 23°C. In that situation, slashing energy costs is as simple as pumping water from 155m underground through a heat exchanger and sending the warmed water 165m back down below via a second bore about 250m away.

This method of exchanging energy with the Earth’s crust is called low-temperature sedimentary aquifer geothermal, and so far it’s keeping everyone happy at the 80-bed Montgomery House nursing home in Perth, described above.

“It uses the thermal stability of the aquifer,” says Dr Ian Brandes de Roos, a principal hydrogeologist with Perth-based Rockwater Consultant Hydrogeologists. “This is low-hanging fruit for low-carbon energy sources for heating and cooling buildings of this size.”

At the Perth aged care home water is pumped from 155m underground and run through this heat exchanger. The warmed water is then sent 165m back down below via a second bore about 250m away

Tapped from underground

Most people don’t think of cold water as an energy source, but it all depends on how you look at a problem, Brandes de Roos says. “The word ‘energy’ trips people up. They’ll say, ‘That’s not energy; that’s just using the thermal inertia of the aquifer.’ But in this instance you are extracting the heat energy from the aquifer that you would otherwise have to supply via a traditional air-conditioning unit.”

The “world class” (in Brandes de Roos’ words) Mullaloo Aquifer also cools the nearby Pawsey Supercomputing Centre, which crunches data for the Square Kilometre Array radio telescope near Geraldton, West Australia.

The dignified sandstone building had been left in a neglected state and was brought back to life by new owner Aegis Aged Care, but heating and cooling these old piles is expensive – with a load estimate of 1,000MWh a year for its automated 800kW HVAC system.

“On a summer’s day you can either be extracting coolth from 40° air and getting that down to 15° for your air-conditioning cycle or you can be getting 22° water and dropping it an extra 6° and using that for your air-conditioning cycle,” Brandes de Roos says. “So it’s significantly more efficient because the temperature is about what your target temperature is.”

The project has been running since July 2018 and modelling suggests $1.5 million in capital and operating expenditure savings over the project’s 30-year life.

Quiet achiever

But just as there are unexpected sources of energy, so there are unexpected benefits when you choose the right one. At Montgomery House, an historic landmark brought back to life, the owners sought a solution that was silent and largely invisible. External evidence of the Rockwater project is only found near a garden bed in the carpark, where a grille covers the production bore.

No-one can hear the pump working away 30m below. The alternative was having an air-handling unit on a heritage building, “which just wouldn’t have been acceptable,” Brandes de Roos says. “With these projects, you don’t hear them and you don’t see them.”

About 10 years ago there were high hopes for geothermal projects where super-hot water drawn from as far as 5km underground would drive turbines for industrial use, but enthusiasm faded over the years despite extravagant investment. There are plenty of successful applications that use the same basic concept on a less ambitious scale, says Brandes de Roos says. The majority of municipal swimming pools in Perth are heated by geothermal energy, for example, with bores running to about 1km.

“Sedimentary basins are the quiet achievers of the geothermal industry and the low-carbon solutions space,” he says. “We see an energy future with a whole bunch of complementary technologies, and geothermal for heating and cooling of larger facilities is a really important part of the mix.”